The present invention relates to mixing a first medium into a second medium. The present invention especially relates to mixing gas to a medium, but it may as well be applied, for example, for mixing liquids, since mixing of gas has considerably higher requirements than the others both on the mixers and mixing methods. The method and apparatus in accordance with the present invention are especially suitable for mixing gaseous bleaching chemicals, such as oxygen or ozone, used in the bleach plants of the wood processing industry, and for the pulp bleaching process applying the mixing method and apparatus in accordance with the present invention. An excellent application is mixing ozone-containing gas with fiber suspension flowing in a pipe and an ozone bleaching process.
The main objective of the present invention is to develop a method of and an apparatus for mixing large volumes of gas into a medium. Further, since the chemical to be added may be extremely rapidly reacting, such as ozone, said preconditions set great demands on the method and apparatus to be developped.
In most of the modern bleaching plants very often large volumes of gas are desired to be mixed into a medium consistency fiber suspension, which means that the consistency of the fiber suspension is approximately 10-18% and it must be possible to mix a large volume of gas therewith. In other words during the mixing process approximately 40 to 80% of the medium is fiber suspension and approximately 20 to 60% gas, the proportion of the gas most usually being approximately 30 to 50%. It is difficult to have a uniform feed of such a large gas volume and to reach a good mixing result, because gas is separated due to local pressure differences to areas of lower pressure, if possible. The non-uniform mixing results on the increase of chemical loss, which further results in a non-uniform bleaching and in poorer runnability of the process.
The use of above mentioned ozone as a bleaching chemical in bleaching will become more and more popular in the future. There is an ongoing transition from pilot testing to applications in a mill scale, which leads to even higher demands on the apparatus due to the characteristic behavior of ozone. Ozone may be produced and used with the modern technique only in very small proportions, whereby most (usually more than 90%) of the chemical to be mixed with the pulp to be bleached is in fact inert carrier gas compared with the ozone. The result is, of course, that the volume of the gas to be mixed is large. Another significant point is that ozone reacts very rapidly with the material in the fiber suspension. Thus the mixing must be at the same time both very quick, efficient and also uniform in result. Since the ozone immediately reacts with all fibrous material it encounters, the ozone-containing gas may not be allowed to meet only a particular portion of a suspension for a single moment, because it will result in a very uneven bleaching. According to the present technology ozone is not at all a selective chemical and it reacts equally efficiently both with the fibrous material and the lignin to be removed or bleached. In other words, if the ozone dosing for a portion of the suspension is excessive, the ozone quickly causes damage also in the suspension, resulting, of course, in poorer quality of the bleached pulp. Thus the mixing must be very uniform right from the beginning. Due to the non-selectivity the ozone cannot also not be overdosed and also not for the reason that ozone is an expensive chemical.
Ozone may be industrially produced only in relatively dilute mixtures. In other words only 5 to 10% of the gas to be fed for the bleaching, is ozone the rest operating merely as a so called carrier gas. The carrier gas is in most cases either oxygen or nitrogen. Therefore, approximately 10 to 20 times the volume of the ozone carrier gas must be fed and mixed although relatively small volumes of ozone are otherwise sufficient for the bleaching.
Some prior art mixers in the use of cellulose industry as well as their applicability in efficient and uniform mixing of large volumes of gas are studied more in detail below.
U.S. Pat. No. 4,416,548 illustrates an embodiment, in which the gas to be mixed is introduced into the frontside of a cylindrical rotor of an apparatus slightly resembling a centrifugal pump to a point where the pulp flowing axially along the suction duct is divided fan-like into a radial flow bringing the gas therewith to the rim of the cylindrical rotor. The flow turns there again axial, flowing, for example, between the pin-like members stationary mounted on the rotor housing and on the outer rim of the rotor to a spiral discharge chamber of the apparatus. The operation of the apparatus is based on that the pin-like members of the rotor pass the members of the housing very close generating a very strong shear force field mixing the chemical effectively to the pulp. The apparatus has two significant defects or disadvantages considering the purpose of the present invention. Firstly, the gas is fed into the center of the rotor into the relatively slowly flowing pulp, which, for example, when feeding ozone, results in a local overdose and damage of cellulose in the particular portion of the pulp. In order to be able to bleach the whole pulp amount, some kind of an overdose should be fed into the mixer, even with a risk of damaging the cellulose. Secondly, there is the disadvantage that subsequent to the efficient "pin mixing zone", pulp is allowed to be quickly discharged into a wider space, a spiral. Consequently, a zone of lower pressure is generated, in which the gas in the pulp is easily separated by the centrifugal force from the fiber suspension, which is still in a fluidized stage. Thus when the consistency of the pulp increases and the pulp forms a plug flow in the discharge channel gas is entrained therewith in large bubbles. As a result therefrom the ozone which possibly still has not reacted in the gas would still react only with the fibers defining the gas bubbles.
FI patent 76132 illustrates a construction which to some extent resembles the arrangement in accordance with the U.S. publication. The apparatus, however, is evidently a centrifugal pump, the impeller vanes of which are arranged two-piece in such a way that it is possible to fit a number of feed and mixing pins for the chemicals between said parts. Thus the feed of chemicals to a strong field of shear forces, is carried out in an orthodox manner but the pulp is discharged from the mixing zone to the spiral of the centrifugal pump, in which the pulp is, as known, subjected to intense centrifugal forces, due to which gas is separated from the pulp to form its own layer. The result is the same as above.
As a third example of the prior art is an arrangement disclosed in U.S. Pat. No. 4,305,894 comprising an axial flow pump, a rotor thereof and mixing means for gas. The liquid, with which the gas is intended to be mixed, is drawn by the rotor to a cylindrical suction duct, into which gas is fed immediately after the rotor in the flow direction through a pipe surrounding the axis of the rotor. A stationary blade is mounted on the outer surface of the pipe immediately after said gas feed to generate together with a liquid, strongly circulating in the duct, a field of shear forces in the liquid so that gas is mixed with the liquid. Additional efficiency to the mixing may be brought about by adding ribs or like on the wall. The apparatus does not, however, guarantee a uniform mixing, because the diameter of the suction duct is rather large and the gas is fed into the center of the flow. It is not possible to ensure that the gas in the apparatus would be able to flow into contact also with the liquid flowing on the outermost layer of the flow, but it is assumed that the gas is mixed with the liquid flowing relatively close to the axis and the liquid in the outermost area of the suction duct remains without gas. When ozone is used, the non-uniform mixing results in non-uniform bleaching result and damage of cellulose because of a local overdose.
A fourth prior art publication worth mentioning is DE 2920337, which generally describes the utilization of the fluidization and different applications thereof, giving an example of mixing liquid or gas into fiber suspension. Said embodiment is illustrated primarily in FIGS. 1-4, of which the construction of FIG. 2 comprises a cylindrical rotor positioned substantially axially in the flow channel, the outer surface of the rotor as well as the inner wall of the flow channel being provided with protrusions. The chemical, gas or liquid, to be mixed is introduced into the rotor through the shaft of the rotor, from the surface of which the chemical is fed to a relatively narrow fluidization zone between the rotor and the wall of the flow channel. It may be assumed of said construction that the mixing of gas into the suspension is uniform, but said apparatus still has some significant defects. Firstly, the illustrated rotor is rather short, which as such is an orthodox arrangement considering the energy consumption and also when the volume of the gas to be mixed is not very great or when the intention is to give no time to the chemical to react with the suspension. However, what is obtained with the mixer described above is a relatively high circumferential speed of the suspension causing the separation of the gas in the area immediately subsequent to the rotor when centrifugal force forces the suspension to the wall of the flow channel and the gas flows to the center of the flow. Said separation tendency in the arrangement according to the publication is so strong also because the axial cross-section of the rotor is rectangular, whereby the pulp being discharged from a rather narrow fluidization zone arrives to a large flow channel. There a local zone with a strong low-pressure effect (i.e. a big pressure difference) is generated and thus gas can readily separate as large bubbles to the center of the flow.
A second embodiment illustrated in DE publication 2920337 (FIGS. 3 and 4) comprises a similar rotor, which is mounted to the flow channel transversely in such a way that the suspension must pass through a narrow gap to the backside of the mixer rotor, where a throttling point of the flow channel is arranged. The mixing process corresponds to the previous embodiment, but is shorter of its duration. The problem is that the suspension is allowed to be discharged from the narrow fluidization zone into a quickly widening channel, whereby gas is able to separate as large bubbles from the suspension.
SE patent document 462 857 discloses a mixer for mixing bleaching agent with pulp. The pulp is tangentially introduced into the mixer housing within which a rotor rotates. The rotor comprises a substantially radial plate provided at its outer edge with an annular mixing member having ribs or grooves lying in a substantially radial plane. In a corresponding manner the stationary housing portions facing said mixing member lie in a substantially radial plane and are provided with ribs or grooves. The pulp together with the bleaching agent travels radially inwardly through the gap between said mixing means and the housing and is axially discharged from the mixer. A characterizing feature of the mixer of the SE publication is that the pulp is discharged through a narrow annular gap into a wide space to be discharged from the mixer.
As is seen from the prior art review above only few previously known apparatuses are able to mix a gaseous chemical quickly and presumably also relatively uniformly into the fiber suspension. Yet, until now, it has always been mixing of relatively small volumes of gas and/or slowly reacting chemicals into the pulp. Thus it is in a way logical that no arrangements in accordance with the prior art can ensure the mixing of large volumes of gases into the pulp so that the gas would also remain uniformly mixed with the pulp subsequent to the mixing.
The objective of the present invention is to eliminate the disadvantages of the prior art apparatuses and to ensure that also large volumes of gases are uniformly mixed with the pulp and that they remain mixed with the pulp also when the pulp is discharged from the mixer to a flow channel or a reaction vessel.
It is possible to further design the mixing apparatus in accordance with the present invention to be applied for carrying out the actual ozone bleaching in such a way that the carrier gas and the possibly excessive ozone are removed by means of the same apparatus and possibly returned by means of a second mixing apparatus to the next bleaching stage or step, whereby the term "displacement bleaching" may well be used. Said displacement bleaching refers to a bleaching, in which a very rapidly reacting chemical is fed into pulp and mixed with the pulp to be treated in a relatively long axial distance (the distance is, of course, affected by the reaction speed of the chemical). The idea, however, is that, when the untreated pulp reaches the mixer, it is mixed with a certain amount of chemicals (in this example ozone), which immediately reacts, leaving no or hardly any reactive chemical in the gas-pulp mixture, only so called carrier gas. When new chemical mixture is fed in the same mixer and mixed into the pulp it pushes the "old" carrier gas from thereahead, which justifies the use of the term "displacement bleaching".
Characterizing features of the method in accordance with the present invention are
to feed gas in the first phase into a fluidized medium flow in a narrow mixing channel; PA1 to dampen the shear force field of the gas-medium-suspension in the second phase and to allow a plug flow to be formed in the medium, whereby the gas remains uniformly distributed in said plug flow. PA1 to feed the bleaching chemical into an annular mixing channel; PA1 to mix the bleaching chemical into a medium which is in a fluidized state; PA1 to further agitate the medium allowing the chemical to react with the medium; PA1 to discharge the medium from the mixing channel to the discharge channel; and PA1 to remove residual chemical from the medium in the discharge channel and/or in the end portion of the mixing channel.
It is again a characterizing feature of the apparatus in accordance with the present invention that the apparatus comprises a mixer body, an inlet channel for medium, a substantially annular mixing channel, a discharge channel and a rotor rotatable in the mixing channel and that the rotor is tapering in the direction of the flow.
It is a characterizing feature of the bleaching process applying the method in accordance with the present invention
The method and apparatus in accordance with the present invention are described more in detail below, by way of example, with reference to the accompanying drawings, in which